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Effect of Low Copper Addition to As-Forged 304 Stainless Steel for Dental Applications

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Effect of Low Copper Addition to As-Forged 304 Stainless Steel for Dental Applications metals Article Effect of Low Copper Addition to As-Forged 304 Stainless Steel for Dental Applications Ker-Kong Chen 1,2,† , Chih-Yeh Chao 3,†, Jeng-Huey Chen 1,2, Ju-Hui Wu 1,4 , Yen-Hao Chang 5 and Je-Kang Du 1,2,* 1 Department of Dentistry, Kaohsiung Medical University Hospital, Kaohsiung 80708, Taiwan; [email protected] (K.-K.C.); [email protected] (J.-H.C.); [email protected] (J.-H.W.) 2 School of Dentistry, College of Dental Medicine, Kaohsiung Medical University, Kaohsiung 80708, Taiwan 3 Department of Mechanical Engineering, National Pintung University of Science and Technology, Pingtung 91201, Taiwan; [email protected] 4 Department of Oral Hygiene, College of Dental Medicine, Kaohsiung Medical University, Kaohsiung 80708, Taiwan 5 PhD Program, School of Dentistry, College of Dental Medicine, Kaohsiung Medical University, Kaohsiung 80708, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-7-3121101 (ext. 7003) † These authors contribute equally to this work. Abstract: The aim of this study was to investigate the effect of incorporating low copper (0, 0.5, 1, 1.5, and 2 wt.%) additions into as-forged AISI 304 stainless steel (304SS). The microstructures and mechanical properties of the steel were examined using scanning electron microscopy and a universal testing machine. The antibacterial properties of the Cu-bearing 304SS specimens were investigated using Escherichia coli. Each specimen was soaked in artificial saliva to detect the release of copper ions through inductively coupled plasma atomic emission spectrometry. The addition of copper had no significant effect on the microstructure of the as-forged Cu-bearing 304SS, but it slightly increased its maximum tensile strength. The antibacterial rate of the as-cast and as-forged 304SS with 2 wt.% Cu was over 80%, which corresponded to an increase in the release of copper ions. This study demonstrates that low-Cu-content stainless steel can reduce bacteria and can be a suitable material Citation: Chen, K.-K.; Chao, C.-Y.; for the oral environment because of the low release of Cu ions. Chen, J.-H.; Wu, J.-H.; Chang, Y.-H.; Du, J.-K. Effect of Low Copper Keywords: Cu-bearing stainless steel; microstructures; mechanical properties; antibacterial properties Addition to As-Forged 304 Stainless Steel for Dental Applications. Metals 2021, 11, 43. https://dx.doi.org/ 10.3390/met11010043 1. Introduction Received: 23 November 2020 Orthodontic appliances, orthodontic wires, and miniscrew implants (MSIs) are com- Accepted: 24 December 2020 mon and are widely used in orthodontic clinics [1,2]. In recent years, MSIs have been Published: 27 December 2020 frequently used as bone anchors for orthodontic treatment. In addition to improving the treatment mode of orthodontics, MSIs can also greatly shorten the treatment time. MSIs are Publisher’s Note: MDPI stays neu- generally made of titanium alloys [3]. Titanium is biocompatible and allows for a high tral with regard to jurisdictional claims bone–implant contact ratio (osseointegration) between dental implants and the host bone in published maps and institutional after healing [4]. From the point of view of the clinician, an ideal MSI requires immedi- affiliations. ate loading after implantation, sufficient mechanical rigidity, and resistance to corrosion. After the orthodontic treatment is completed, the MSI must be easy for the clinician to remove without breaking. However, the high degree of osseointegration required for dental Copyright: © 2020 by the authors. Li- implants is not a requirement for orthodontic MSIs to function as anchorage devices [5]. censee MDPI, Basel, Switzerland. This Unlike titanium alloys, stainless steel (SS) screws tend to develop primary stability (a article is an open access article distributed mechanical locking interface) between the screw and bone, which lowers the removal under the terms and conditions of the torque, leading to easier retrieval [6]. Creative Commons Attribution (CC BY) There are four major differences between MSIs and general dental implants. One is license (https://creativecommons.org/ that MSIs placed in the alveolar bone must be partially exposed outside the oral mucosa, licenses/by/4.0/). which increases the risk of infection. Second, MSIs need to be immediately loaded and Metals 2021, 11, 43. https://dx.doi.org/10.3390/met11010043 https://www.mdpi.com/journal/metals Metals 2021, 11, 43 2 of 8 placed in the alveolar bone. Third, the direction of the force is usually lateral, and the implant needs to be removed after the orthodontic treatment is completed. Fourth, MSIs are usually small in size and diameter. The use of MSIs requires that the existence of multiple microorganisms in the oral cavity be considered. Colonies of these microorganisms form a biofilm in the oral cavity, which can easily cause bacteria to accumulate and attach to the surfaces of instruments and prostheses, causing inflammation and infection of the soft tissues [7]. Some scholars have pointed out that tissue inflammation, infection, and peri-implant inflammation may increase the failure rate of MSI implantation by 30% in its initial stage [8]. To solve these problems, stainless steel can acquire antibacterial functions through methods including surface treatment [9], amorphous treatment [10], and alloy design [11]. However, as surface spraying can only result in short-term antibacterial effects, alloy design is a promising method for obtaining long-term antibacterial properties [11]. Copper exists in trace amounts in the human body (100–200 mg) [12]. The upper limit of daily copper intake in the human body is 0.7–2 mg/d, depending on the age group [12]. A moderate amount of copper can promote the production of collagen, bones, blood vessels, and skin in the human body [11]. However, an excessive intake of copper causes cell damage or death, which harms the human body and causes diseases such as Wilson’s disease and Alzheimer’s disease [13]. Although copper-containing stainless steel has good mechanical and antibacterial properties, it has development potential in the field of biomedical materials that is yet to be exploited. However, the release of copper ions should still be controlled so as to ensure the safety of copper-containing stainless steel for human use. Furthermore, copper has been proven to be an effective antibacterial element [14–17]. Therefore, many studies have been conducted on copper-containing stainless steel. However, because copper is a stable element of austenitic iron in stainless steel, it has a grain refinement effect. Therefore, the addition of copper can strengthen the mechanical properties of stainless steel [18]. In terms of improving the mechanical properties of stainless steel, forging [18] and heat treatment [19] are also used to meet the strength requirements in stainless steel for use in different types of medical equipment. Austenitic stainless steel has numerous excellent properties, such as nonmagnetism, high strength, good corrosion resistance [20], and superior workability [12]. It is widely used in industrial applications and is suitable for biomedical applications. The AISI 304 alloy is one of the most popular stainless steels. Zhang et al. [11] added 3.9 wt% copper to 304 stainless steel. The experimental steel was solution treated and then aged so as to obtain Cu-rich phase precipitation in the matrix, which afforded the steel a strong antibacterial function against Porphyromonas gingivalis (P. gingivalis). However, the corrosion resistance of copper-containing stainless steel is more controversial. Some studies have reported that copper-added stainless steel is prone to local corrosion phenomena [13], such as pitting corrosion [21], stress corrosion [22], and grain boundary corrosion [23]. Tong et al. [24] added 2.5–3.5 wt% copper to 316 stainless steel. Their experimental results showed that 316 copper-containing stainless steel could improve mechanical strength after solid solution and aging heat treatment. However, the corrosion resistance of 316 stainless steel that contains copper is worse than that of pure 316 stainless steel. Although the precipitation of the Cu-rich phase can improve the antibacterial properties, Hong et al. [14] indicated that the presence of copper precipitates affects the stability of the passive film on the surface of stainless steel and reduces its corrosion resistance. Therefore, the copper content of current, common copper-containing austenitic stainless steel is approximately 3 to 5 wt% [16,24–26]. Even though the biocompatibility of 5 wt% copper with stainless steel is within the allowable range, there are still concerns about excessive dissolution in the oral cavity in the long term. The intake of excessive copper leads to severe liver damage (cirrhosis), owing to copper-induced oxidative damage in the liver and other tissues [27]. Therefore, in this study, we mainly investigated the antibacterial and mechanical properties of forged 304 Metals 2021, 11, 43 3 of 8 stainless steel by adding a low copper content (0.18, 0.54, 1.05, 1.80, and 2.05 wt%). We also measured the release of copper ions by soaking the materials in artificial saliva. 2. Materials and Methods 2.1. Specimen Preparation In this study, 0.18, 0.54, 1.05, 1.80, and 2.05 wt% copper was added to austenitic 304 stainless steel (Table1), which was provided by the Datian Precision Industry Co., Ltd. (O-TA Company, Shenzhen, China). The alloy was melted in a vacuum melting furnace (Ultimate Materials Technology Co., Ltd., Hsinchu, Taiwan)at 1700 ◦C. The smelted stainless steel block was homogenized at 1500 ◦C for 1 h. In addition, forging at 1300 ◦C was performed to produce plates with a thickness of 3 ± 0.1 mm, which was followed by solution treatment and water cooling at 880 ◦C for 1 h. The resulting castings and forgings were all made into standard square tensile test bars of 10 × 10 × 2 mm3 so as to facilitate the subsequent experiments. Table 1. Chemical composition (wt%) of the experimental Cu-bearing 304 stainless steel.
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